Abstract: In sun simulators of the off-axis type, the angle at which the collimator mirror reflects the incident beam onto the test object causes a distortion error dependent on the radius of curvature of the collimator mirror. This error distorts the cross-section of the beam in the test plane. An optical arrangement is disclosed and includes an integrator assembly having a field lens array made up of a plurality of field lenses arranged one next to the other. The distortion is compensated for by providing that the periphery of each lens is elliptical with the axes ratio of the ellipse chosen in accordance with the magnitude of the previously computed error of distortion.